DNA Replication
In order for cells to reproduce, DNA replication is necessary. This is a process where a parental DNA strand gives rise to two daughter DNA strands. This is a very complex process, so do not hesitate to ask your teacher for clarification when needed. For our intents and purposes, the replication process below is for prokaryotes. The process for eukaryotes is similar but more complicated.
1. Helicase unzips the parental DNA by splitting the hydrogen bonds between nitrogenous bases at the origin of replication. This creates two single "template" strands. Prokaryotes only have one origin of replication whereas eukaryotes have hundreds or thousands due to their larger genomes.
2. Binding proteins (also called clamps) pin these two template strands in place.
3. Topoisomerase reduces the twisting tension in the region in front of the replication forks.
4. Primase makes RNA primers, which allows DNA polymerase to start base pairing.
5. DNA Polymerase III adds complementary bases in the 5' to 3' direction. Since DNA strands are antiparallel, if the template strand goes from the 3' to 5' direction (left to right), then the new strand goes from 5' to 3' (left to right). Also, DNA polymerase works faster in prokaryotic organisms than in eukaryotic organisms.
6. If the DNA polymerase is adding bases in the same direction as the direction that the helicase is going, then a long continuous strand is being created (called the leading strand). If the DNA polymerase is going in the opposite direction as the helicase, then a lagging strand is being created. The lagging strand is characterized by short, discontinuous chains of nitrogenous bases called Okazaki fragments. This is because as the helicase unzips the parental DNA strand, more and more template strands are exposed behind the DNA polymerase, which is going in the opposite direction as the helicase. In order to pair bases with the newly exposed template strand, a new DNA polymerase must start further back in the 5' direction. However, this leaves small gaps between where one DNA polymerase starts to pair and where another one finishes.
7. DNA Polymerase I replaces the RNA primers with DNA, however, it leaves a gap in the sugar-phosphate backbones between the last DNA nucleotide that it adds and the first DNA nucelotide that is added from the RNA primer that it just removed. (In addition, there would be no DNA nucleotides in the very end of the complementary strand's 5' end. This is because there is no 3' end behind it for DNA Polymerase I to attach to. Thus, every replication makes the DNA strand shorter. Later we will discuss the effects of such shortening, as well as measures to counter it.)
8. The aforementioned gaps are connected by DNA ligase.
9. DNA polymerase proofreads the newly created DNA strand for errors. If one is detected, it will replace the wrong base with the correct one. Occasionally errors may remain undetected, and this can cause a mutation to occur.
10. Finally, two new daughter DNA strands are made.
1. Helicase unzips the parental DNA by splitting the hydrogen bonds between nitrogenous bases at the origin of replication. This creates two single "template" strands. Prokaryotes only have one origin of replication whereas eukaryotes have hundreds or thousands due to their larger genomes.
2. Binding proteins (also called clamps) pin these two template strands in place.
3. Topoisomerase reduces the twisting tension in the region in front of the replication forks.
4. Primase makes RNA primers, which allows DNA polymerase to start base pairing.
5. DNA Polymerase III adds complementary bases in the 5' to 3' direction. Since DNA strands are antiparallel, if the template strand goes from the 3' to 5' direction (left to right), then the new strand goes from 5' to 3' (left to right). Also, DNA polymerase works faster in prokaryotic organisms than in eukaryotic organisms.
6. If the DNA polymerase is adding bases in the same direction as the direction that the helicase is going, then a long continuous strand is being created (called the leading strand). If the DNA polymerase is going in the opposite direction as the helicase, then a lagging strand is being created. The lagging strand is characterized by short, discontinuous chains of nitrogenous bases called Okazaki fragments. This is because as the helicase unzips the parental DNA strand, more and more template strands are exposed behind the DNA polymerase, which is going in the opposite direction as the helicase. In order to pair bases with the newly exposed template strand, a new DNA polymerase must start further back in the 5' direction. However, this leaves small gaps between where one DNA polymerase starts to pair and where another one finishes.
7. DNA Polymerase I replaces the RNA primers with DNA, however, it leaves a gap in the sugar-phosphate backbones between the last DNA nucleotide that it adds and the first DNA nucelotide that is added from the RNA primer that it just removed. (In addition, there would be no DNA nucleotides in the very end of the complementary strand's 5' end. This is because there is no 3' end behind it for DNA Polymerase I to attach to. Thus, every replication makes the DNA strand shorter. Later we will discuss the effects of such shortening, as well as measures to counter it.)
8. The aforementioned gaps are connected by DNA ligase.
9. DNA polymerase proofreads the newly created DNA strand for errors. If one is detected, it will replace the wrong base with the correct one. Occasionally errors may remain undetected, and this can cause a mutation to occur.
10. Finally, two new daughter DNA strands are made.